1. Field of the Invention
This invention relates generally to an electrophotographic printing system, and more particularly concerns an apparatus and method for controlling charging of a photoconductive member.
2. Description of the Prior Art
The basic xerographic process comprises exposing a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document. A development system, thereupon, moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. Thereafter, the toner powder image is transferred to a sheet of support material. The sheet of support material then advances to a fuser which permanently affixes the toner powder image thereto.
Before the photoconductive member can be exposed to a light image, the photoconductive member must be charged by a suitable device. This operation is typically performed by a corona charging device. One type of corona generator consists of a current carrying wire enclosed by a shield on three sides and a wire grid over and spaced apart from the open side of the shield. A uniform potential is applied to the wire and the wire grid. Electrostatic fields develop between the charged wire and the shield, between the wire and the grid, and between the charged wire and the (grounded) photoconductive member. Electrons are repelled from the wire and the shield resulting in a charge at the surface of the photoconductive member. The wire grid, located between the wire and the photoconductive members, because of the field between the grid and the wire, helps control the charge strength and uniformity on the photoconductive member caused by the other aforementioned fields.
The control of the charge strength and uniformity on the photoconductive member is very important because consistent high quality reproductions are best produced when a uniform charge is obtained on the photoconductive member. If the photoconductive member is not charged to a sufficient potential, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of development material will be correspondingly lessened. As a result, the copy, produced therefrom, will be faded. If, however, the photoconductive member is overcharged, the converse will occur and too much developer material will be deposited on the photoconductive member. As a consequence thereof, the copy produced therefrom, will have a gray or dark background instead of the white background of the copy paper. Areas intended to be gray are black. Tone reproduction is poor. Additionally, if the photoconductive member is overcharged too much, the photoconductive member can be permanently damaged.
In a typical xerographic charging system, the amount of voltage obtained at the point of electrostatic voltage (ESV) measurement of the photoconductive member is less than the amount of voltage applied at the point of charge application. In addition, the amount of voltage applied to the corona generator required to obtain a desired constant voltage on the photoconductive member must be increased or decreased according to various factors which affect the photoconductive member. Such factors include the rest time of the photoconductive member between printing, the voltage applied to the corona generator for the previous printing job, the copy length of the previous printing job, machine to machine variance, the age of the photoconductive member and changes in the environment.
Historically, the only factor corrected in applying a voltage on the corona generator to obtain a uniform voltage at the photoconductive member was a rest recovery correction factor. The rest recovery factor attempted to correct for the fact that the photoreceptor responds to charges differently after it is allowed to rest at which time no charge is applied. Preferably, the manner of adjusting the voltage at the corona generator was to adjust the voltage applied to the wire grid.
For example, it would not be uncommon at the end of a 200 copy job for the corona charging device of a copier to generate 1200 volts to obtain 900 volts at the point of measurement on the photoconductive member as measured by an electrostatic voltmeter. After allowing the copier to remain idle for 15 minutes, the corona generator might then need to put out only 1000 volts to obtain 900 volts on the photoconductive member.
The classical rest recovery correction factor can be written as: ##EQU1## where Percentage of Recovery=A+B natural log (rest time), in which A and B were predetermined constants.
Although the classical rest recovery factor has proven beneficial in the control of the charge strength and uniformity on a photoconductive member, there is a need to correct the great many factors which affect the charge strength and uniformity on a photoconductive member.
The problems with typical xerographic charging control systems are not limited to the difficulties associated with rest recovery. In a typical charge control system, the point of charge application, and the point of charge measurement is different. The zone between these two devices loses the immediate benefit of charge control decisions based on measured voltage error since this zone is downstream from the charging device. This. zone may be as great as a belt revolution or more due to charge averaging schemes. This problem is especially evident in aged photoreceptors because their cycle-to-cycle charging characteristics are more difficult to predict. The problem results in improper charging, often leading to early photoreceptor replacement. Thus, there is a need to anticipate what the next cycles behavior will be and compensate for it beforehand.
The following disclosures may be relevant to various aspects of the present invention:
U.S. Pat. No. 2,956,487, Patentee: E. C. Giaimo, Jr., Issued: Oct. 18, 1960; PA1 U.S. Pat. No. 3,335,274, Patentee: Codichini et al., Issued: Aug. 8, 1967; PA1 U.S. Pat. No. 3,469,351, Patentee: Cunningham, Jr., Issued: Feb. 17, 1970; PA1 U.S. Pat. No. 3,604,925, Patentee: Snelling, Issued: Sep. 14, 1971; PA1 U.S. Pat. No. 3,688,107, Patentee: Schneider et al., Issued: Aug. 29, 1972; PA1 U.S. Pat. No. 3,699,388, Patentee: Ukai, Issued: Oct. 17, 1972; PA1 U.S. Pat. No. 3,934,141, Patentee: Vargas, Jr., Issued: Jan. 20, 1976; PA1 U.S. Pat. No. 3,935,532, Patentee: Shuey et al., Issued: Jan. 27, 1976; PA1 U.S. Pat. No. 4,435,677, Patentee: Thomas, Issued: Mar. 6, 1984; PA1 U.S. Pat. No. 4,502,777, Patentee: Okamoto et al., Issued: Mar. 5, 1985; PA1 U.S. Pat. No. 4,512,652, Patentee: Buck et al., Issued: Apr. 23, 1985; PA1 U.S. Pat. No. 4,796,064, Patentee: Torrey, Issued: Jan. 3, 1989; PA1 U.S. Pat. No. 4,806,980, Patentee: Jamzadeh et al., Issued: Feb. 21, 1989; PA1 U.S. Pat. No. 4,920,380, Patentee: Ueda et al., Issued: Apr. 24, 1990; PA1 U.S. Pat. No. 4,935,777, Patentee: Noguchi et al., Issued: Jun. 19, 1990; PA1 U.S. Pat. No. 4,939,542, Patentee: Kurando et al., Issued: Jul. 3, 1990; PA1 U.S. Pat. No. 4,970,557, Patentee: Masuda et al., Issued: Nov. 13, 1990; PA1 U.S. Pat. No. 5,003,350, Patentee: Yui et al., Issued: Mar. 26, 1991.
The relevant portions of the foregoing disclosures may be briefly summarized as follows:
U.S. Pat. No. 4,796,064 discloses a control device for adjusting the surface potential of an image bearing member during the initial cycles of a job run wherein the image bearing member manifests varying characteristics after completion of a job run. The control device includes logic circuitry having means to predict changed characteristics of the image bearing member after completion of a first job run at the initiation of a second job run and means to determine a relationship between a charging current of a charging member and a measured surface potential of the image bearing member. More specifically, the control device predicts the charging characteristics of the image bearing members as a function of a rest recovery and a cumulative sum of previous jobs.
U.S. Pat. No. 4,512,652 discloses an electrophotographic printing machine wherein a controller regulates charging of a photoconductor member according to stored information. The controller determines a charging current as a function of a "start of day" charging current, a previous operating cycle charging current, and/or a rest time between successive copying cycles.
U.S. Pat. No. 4,806,980 discloses a feedforward process control for an electrophotographic machine wherein an initial voltage level and an exposure level are process control parameters of the machine. Signals are produced and stored having values characteristics of: (1) a level of at least one of the parameters; and (2) a bias voltage level. A comparison signal is produced by comparing the signal values of charges and the sensed parameters associated with the latent images with the stored signal values for the corresponding latent charge images. Compensation algorithms are used to compensate for noise and disturbances in the initial charge. The feedforward process control acts in an anticipatory manner before the effect of the noise and disturbances affects the results.
U.S. Pat. No. 4,939,542 discloses an image forming apparatus having: (1) a memory means for storing a measured value of a surface potential of a photoreceptor drum obtained by a potential sensor; and (2) a charger-output control means for controlling an output from a charger, based on the measured value stored in the memory means. The charger-output control means obtains a value at the surface potential of the photoreceptor which is measured by the potential sensor at a time when a voltage from a voltage generation circuit is applied to the photoreceptor by operating a switching means, estimated by an arithmetic operation based on the measured value obtained. The output charger, at the next series of image forming operations, is adjusted to be equal to the measured value that has been read out by the potential sensor.
U.S. Pat. No. 4,502,777 discloses an electrophotographic copying apparatus which includes: (1) a device for detecting conditions affecting the operating characteristics of a photoreceptor; (2) a device for determining a state of operation of an image forming device according to the conditions detected by the detecting device; (3) a device for correcting the state of operation of an image forming device so as to render a potential of a latent image formed on a photoreceptor surface; and (4) a device for revising a reference equation based on the conditions detected by the detecting device and the state of operation which has been corrected by the correcting device. The conditions have a predetermined relationship which are represented by a predetermined reference equation.
U.S. Pat. No. 4,435,677 discloses a power regulating device which maintains a constant rms voltage across a load by periodically interrupting an application of voltage to the load at a predetermined number of cycles. A function solution to a equation is incorporated into the device which describes a relationship between the rms voltage developed across the load and rms voltage of a desired control set point. The solution of the equation is monitored so as to reach a fixed value. When a fixed value is reached, a primary current flow to the load is interrupted for a predetermined number of half or full cycles.
U.S. Pat. No. 4,920,380 discloses a method for controlling electric potential on the surface of a photoconductive member. The electric potential of a photoconductive member is always maintained at a certain value by controlling a charge output of a charging means at a predetermined value. After a long period of suspended operation, an initial charge output is lowered according to the length of the suspended operation. Subsequently, the charged output is gradually increased to a predetermined value so that the surface potential of the photoconductive member is always maintained at a specific constant value.
U.S. Pat. No. 4,935,777 discloses a method of stabilizing surface potential of a charged photoreceptor wherein a level of exposure of charge removing light is modified according to fatigue and recovery characteristics of the photoreceptor. During a continuous operation of the photoreceptor, the level is logarithmically reduced, and after a rest period, the initial level of exposure is logarithmically increased as a function of the length of the rest period and the level of exposure prior to the rest period.
U.S. Pat. No. 4,970,557 discloses a method for controlling image quality for an electrophotographic process according to the duration of a rest period and a cumulative copy count. The speed of development of the electrophotographic apparatus is decreased with increasing rest period duration and is increased as the cumulative copy count increases.
U.S. Pat. No. 5,003,350 discloses a method for controlling a voltage applied to a charging grid for charging a photoreceptor. The voltage is controlled as a function of either the number of rotations or the rotation time of the photoreceptor in order to maintain the voltage of the photoreceptor at a constant level.
U.S. Pat. No. 3,935,532 discloses an electrometer system particularly adapted for non-contact measurement of electrostatic charges in electrostatography, such as the charge level on photoreceptor surface areas in xerographic machines. The electrometer circuit disclosed therein may be used for automatic diagnostics or automatic control of one or more xerographic processing elements.
U.S. Pat. No. 3,335,274 discloses a xerographic charging apparatus with means to automatically control the potential applied to a corona wire. Through the automatic control of the potential of the corona wire, a uniform electrostatic charge may be deposited on a xerographic plate.
U.S. Pat. No. 3,604,925 discloses an apparatus for automatically controlling the amount of electrostatic charge applied to a plate by controlling the potential applied to a corona wire. An electrical circuit in a corona generating device is utilized to deposit a uniform charge on a xerographic plate.
U.S. Pat. No. 3,496,351 discloses a control circuit for a corona charging device for use in charging the xerographic plate in a stepping xerographic apparatus whereby a uniform electrostatic charge is applied to the xerographic plate at any stepping rate of the xerographic plate.
U.S. Pat. No. 2,956,487 discloses a method and means for controlling the steps of electrostatic printing. Controlling means can produce a control signal which may used to control the magnitude of electrostatic charge produced on a photoconductive coating. A voltage source is varied by varying the voltage applied to a grid closely spaced between the wires of a corona discharge apparatus and the photoconductive coating.
U.S. Pat. No. 3,934,141 discloses an apparatus for automatically regulating the amount of charge applied to an insulating surface such as a photoreceptor. An electrometer, for measuring the electrostatic potential on the insulating surface, is utilized to generate an error signal. In response to the error signal, the magnitude of the voltage applied by the power supply to a corona electrode is varied. The variation in the voltage magnitude causes the wire to apply sufficient charge to the insulating surface to reduce the error signal to substantially zero.
U.S. Pat. No. 3,688,107 discloses an electrical configuration for a corona generating device whereby a uniform electrostatic charge may be rapidly deposited on an electrostatographic plate.
U.S. Pat. No. 3,699,388 discloses an electrostatic charging apparatus having a means to maintain the magnitude of the discharge field thereof constant. A detection electrode is positioned in the field of the corona discharge and is connected via a resistor and amplifier to the power source to control the same in accordance with the detected corona discharge.